The present invention relates generally to magnetic transducers for reading information signals recorded in a magnetic medium and, more particularly, to a magnetoresistive read transducer based on the spin valve effect wherein at least the magnetoresistive element sensitive to an applied magnetic field includes a back layer of a nonmagnetic, conducting material.
It is well known in the prior art to use a magnetic read transducer referred to as a magnetoresistive (MR) sensor or head for reading data from a magnetic storage medium surface at great linear densities. An MR sensor detects magnetic field signals through the resistance changes of a read element fabricated of a magnetic material as a function of the strength and direction of magnetic flux being sensed by the read element. These prior art MR sensors operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine (cos.sup.2) of the angle between the magnetization and the direction of sense current flow through the element. A more detailed description of the AMR effect can be found in "Memory, Storage, and Related Applications", D. A. Thompson et al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).
More recently, a different, more pronounced magnetoresistance has been observed resulting from the effects of spin-dependent-scattering of conduction electrons in magnetic multilayered structures of the form (F/NM).sub.n where F is a ferromagnetic metal and NM is a nonferromagnetic metal. This effect has been found in a variety of systems such as sputtered Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures of the form (F/NM/F) in which the magnetization orientation in one of the ferromagnetic layers is fixed by exchange anisotropy. The physical origin of the magnetoresistance is the same in both types of structures: the application of a magnetic field causes a variation in the relative orientation of the magnetizations of adjacent ferromagnetic layers. The resistance of the structure changes as the alignment of the magnetizations changes from parallel to antiparallel. This mechanism produces a magnetoresistance that, for selected combinations of materials, is greater in magnitude than the AMR, and is referred to as the "spin valve" magnetoresistance (SVMR) or giant magnetoresistance (GMR).
U.S. Pat. No. 4,949,039 to Grunberg describes a layered magnetic structure which yields enhanced MR effects caused by antiparallel alignment of the magnetizations in the magnetic layers, As possible materials for use in the layered structure, Grunberg lists ferromagnetic transition metals and alloys, but does not indicate preferred materials from the list for superior MR signal amplitude. Grunberg further describes the use of antiferromagnetic-type exchange coupling to obtain the antiparallel alignment in which adjacent layers of ferromagnetic materials are separated by a thin nonmagnetic interlayer of chromium (Cr) or yttrium (Y), for example.
U.S. patent application Ser. No. 07/625,343 filed Dec. 11, 1990, now U.S. Pat. No. 5,206,590 assigned to the instant assignee, discloses an MR sensor in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetizations of the two layers and which is independent of the direction of current flow through the sensor. This structure produces a magnetoresistance that is based on the spin valve effect and, for selected combinations of materials, is greater in magnitude than the AMR.
U.S. Pat. No. 5,159,513, issued Oct. 27, 1992, assigned to the instant assignee, discloses an MR sensor based on the above-described spin valve effect which includes two thin film layers of ferromagnetic material separated by a thin film layer of a non-magnetic metallic material wherein at least one of the ferromagnetic layers is of cobalt or a cobalt alloy. The magnetization of the one ferromagnetic layer is maintained perpendicular to the magnetization of the other ferromagnetic layer at zero externally applied magnetic field by exchange coupling to an antiferromagnetic layer.
In order to meet the requirement for ever increasing data storage densities in today's magnetic storage systems, it is required that the magnetic flux sensing elements in MR heads be fabricated in progressively thinner layers of ferromagnetic material. MR sensors utilizing ultrathin magnetic flux sensing layers, about 15 A, for example, exhibit degraded MR coefficients both for the conventional AMR sensors as well as for the more recent SVMR or GMR sensors.